Add Boltz blog post

docs/_blog/boltz-payjoin.md

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+---
+title: "Swap on-chain funds into a Lightning channel with Payjoin"
+description: "Boltz combines Submarine Swaps with Payjoin to privately transfer on-chain funds into a Lightning channel, while saving on network fees."
+date: "2025-04-10"
+authors:
+  - spacebear
+tags:
+  - privacy
+  - integrations
+---
+
+[Boltz](https://boltz.exchange) is a non-custodial bridge that allows users to
+trustlessly pay a Lightning invoice with an on-chain payment, via
+[Submarine Swap](https://www.voltage.cloud/blog/submarine-swaps-how-do-they-work-and-where-they-are-used).
+
+![Boltz Submarine Swap](../assets/boltz-swap.png)
+
+As a result, the on-chain Boltz wallet ends up accumulating many small UTXOs that
+must be routinely managed and consolidated to avoid high transaction fees.
+
+## Enter Payjoin
+
+Payjoin enables the Boltz receiver wallet to add its own input(s) to an incoming
+Submarine Swap transaction, thus consolidating UTXOs at the time of payment.
+
+![Boltz Submarine Swap with Payjoin](../assets/boltz-payjoin-swap.png)
+
+The sender pays the transaction fee for this consolidation, and gets better
+privacy in return. An outside observer can't know with certainty which input
+belongs to the customer vs. which belong to Boltz, and the resulting
+consolidation obfuscates the payment amount. Such a transaction also breaks the
+[common-input heuristic](https://en.bitcoin.it/wiki/Common-input-ownership_heuristic),
+weakening the assumption that all inputs in a transaction belong to the same
+sender and providing much better privacy properties to every participant in the
+Bitcoin network.
+
+![Payjoin breaks the common-input heuristic](../assets/boltz-cih.png)
+
+## Proof of concept
+
+For the MIT Bitcoin hackathon, the Payjoin team implemented a proof of concept
+by integrating the Payjoin Dev Kit into the
+[Boltz Backend](https://github.com/DanGould/boltz-backend).
+
+<video width="100%" controls><source src="../assets/boltz-payjoin.mp4">No video support.</video>
+
+This demo in the span of a 30 hour hackathon alongside
+[two](https://x.com/payjoindevkit/status/1909667705730269274)
+[other](https://github.com/thebrandonlucas/interactive-payjoin)
+POC integrations.
+This achievement highlights how far PDK has come in making Payjoin accessible
+and ergonomic for developers to use.
+
+## Next Steps
+
+Together, these POCs represent an early vision of a future where Payjoin is
+widely supported and easy to implement. The first step is to get this integration
+production-ready and well-tested. Progress on this work can be tracked on
+[this Boltz PR](https://github.com/BoltzExchange/boltz-backend/pull/892).